Cathode ray tubes (CRTs) are the most widely used electro-optic image transducers ever devised. Their major usage began with the introduction of television news and entertainment broadcasting and has now spread into all systems of visual image presentation information displays and computer data readout.
In contrast with electrostatically deflected cathode ray tubes which are employed in oscillographic instrument displays, the more common television type line-by-line (raster) screens are generated by rapid movement of the information modulated electron beam by means of magnetic fields introduced by coils mounted externally on the neck of the tube. An accelerating voltage of several thousand volts assures projection of the electron beam onto the phosphor coated screen with sufficient energy to cause it to write with a brilliant glow.
The art of electromagnetic deflection of cathode ray beams is well described in an early disclosure of Otto Schade wherein crossed magnetic fields are produced by current flow in inductors positioned on opposite sides of the CRT neck. Schade, Magnetic-Deflection Circuits for Cathode-Ray Tubes, RCA Rev., VIII, No. 3, P506, September 1947. These "deflection coils" apply motor forces to the electron stream thereby causing it to be deflected so that its point of termination on the phosphor screen surface appears as a line trace due to the pesistence of vision of the observer. When the beam is deflected according to an orderly procedure of movement such as from left to right across the screen starting at the upper left and proceeding downward, a rectangular patch is recorded for the viewer's visual retention. If this procedure is repeated often enough the illusion of continued presence of such an image field is created. Variations in the intensity of the electron beam as it moves repeatedly through such a scanned raster register in the eye and mind of the viewer as recognizable geometric images.
It is common practice to move the electron beam across the screen at a uniform velocity both in the direction of the rapid line scan (generally horizontal) as well as in the slower field scanning direction. Linearly variant current ramps are applied to the deflecting coils on the neck of the tube to accomplish this. Each current ramp terminates as the electron beam reaches its extreme position and then reverses direction quickly causing the beam to return to its starting point. In order to support the illusion of continued image presence without tell-tale flicker becoming apparent, it is necessary to refresh the entire image at least fifty or sixty times per second. When a fine structure of many hundred scanning lines are to be included in the display so as to provide good image resolution, the line scanning rate may need to be as high as one hundred thousand sweeps per second although it is standard practice to use only about sixteen thousand for broadcast television purposes.
The cathode ray beam accelerating voltage (second anode voltage) employed in modern equipment ranges from twelve to twenty-five kilovolts depending upon screen size and the desired brightness level of the image generated. The current requirement imposed upon this high voltage source may range from fifty or one hundred microamps for monochrome screens to one or two milliamperes for color cathode ray tubes. The high voltage source itself can be any available type of power generator provided it can support the current demand of the CRT and has an upper current limit of a few milliamperes beyond which the voltage collapses so as to protect servicing personnel from instant electrocution should they make accidental contact therewith.
Early in the development of television broadcasting a unique type of second anode voltage supply for magnetically deflected cathode ray tubes was described by A. W. Friend and later improved upon by many others. Fried, Television Deflection Circuits, RCA Rev., March, 1947 p. 98 (condensed in Television, Vol. V, pg. 170). This very simple and inexpensive high voltage power supply system is a supplement to the magnetic beam deflection technique described above and makes use of the rapid current reversal which occurs in the line sweep deflection coil. This reversal rapidly and repeatedly returns the electron beam to its start of sweep position. It turns out moveover that whenever an electric current changes value rapidly while flowing in an inductor such as a deflecting coil it creates a substantial voltage across the terminals of the coil by self induction. The relationship between an inductor of some value, L (Henrys), the voltage E across it, and the rate of change of current flowing in it (di/dt) (amps per second) is exposed as E=-L di/dt. Thus a brief voltage pulse reaching several hundred volts in magnitude is generated on the terminals of the deflection coil as it causes "flyback" of the CRT beam, line-by-line.
As a source of voltage, this "flyback" pulse found on the deflection coil terminals is powerful and can stand heavy loading due to the low internal impedance of the deflection coil system. It is only necessary therefore to "step up" this voltage by means of a transformer. The several hundred volt level of the deflection coil can thus be raised to the several thousand volt (rectified dc) level required to supply second anode voltage for the tube.
In this manner a low cost combination line deflection and high voltage generator has been developed for use with cathode ray tubes. Fortunately the system has its own current limitations which render it safe to servicing personnel. The flyback high voltage system has reached almost universal acceptance in all forms of commercial cathode ray display devices in spite of one important limitation. The latter being a visible geometric expansion of the raster area as screen brightness is increased. This defect is due to falling second anode voltage or poor high voltage regulation. This effect is now considered totally unacceptable for such critical display systems as ultrasonic medical imagery, word processing business machines, computer aided design screens, and other systems which are sensitive to exactness of image size.
The term voltage regulation is defined as the ratio of the change in a power supply's voltage before and after current loading is applied to it expressed as a percentage. It was known from the beginning that a "flyback" type high voltage power supply would have rather poor voltage regulation, typically ten to fifteen percent. This was thought to be satisfactory however for entertainment television display purposes; in view of its simplicity, low cost and the safety features of this system.
The lack of good voltage regulation in the flyback-type high voltage described above is due to the inherently high internal source impedance of the step up transformer and pulse rectifying arrangement. The system operates in a completely open loop manner, and generally lacks the benefit of any current preload. Many prior art attempts have been made to stabilize the voltage of these power supplies, most of which are tabulated below:
1. The use of a high voltage shunt thyristor (for example a Zener diode) to load the supply until an external load is applied thereby holding a fixed threshold voltage level. This approach wastes power and heats up the thyristor which leads to early destruction.
2. The use of a vacuum tube shunt regulator with or without voltage feedback loop control. This approach is also a power waster because the tube heater must also be activated. However, the components exhibit longer life.
3. The use of an adjustable flyback interval under feedback loop control using either inductive or capacitive tuning means to control flyback voltage. Such a technique requires complex adjustment and the variable tuning is generally visible on the screen.
4. The use of a separate flyback high voltage supply section independent of the deflection yoke section with an adjustable dc voltage source under loop feedback control. Although this is successful and is widely used, it is expensive and power inefficient due to near duplication of components.
5. The use of a supplementary adjustable dc source added to the high voltage winding to offset internal drop as it occurs. This also requires a large number of additional components and power is inefficient.
Finally, while the above problem is characteristic of flyback-type high voltage power supplies it is symptomatic of all pulsed voltage power supplies.